A number of impact sensors have heretofore been proposed for activating vehicle restraint systems and other similar applications. For example, U.S. Pat. No. 4,329,549 discloses a sensor in which a spherical ball is biased toward one end of a cavity by a permanent magnet positioned externally of the cavity. Severe deceleration of a vehicle, on which the sensor is mounted and appropriately oriented, exerts sufficient force on the ball to overcome the force of attraction to the magnet, and to propel the ball longitudinally through the cavity against a pair of switch contacts. The switch contacts are connected to suitable devices for inflating an air bag or the like to prevent impact of a vehicle occupant against the steering wheel or dashboard.
A disadvantage of this design is that closely controlled tolerances are required between the ball and the surrounding cavity, which are difficult and expensive to control in manufacture. Furthermore, the ball must be sufficiently large to generate reliable contact force against the switch elements, necessitating use of a large magnet and consequently decreasing responsiveness of the sensor to impact forces. Weight of the ball may be reduced by plating the switch contacts with gold or the like, increasing the cost of manufacture. Dust particles and the like decrease reliability of the mechanical switch contacts. Additionally, the sensor disclosed in this patent is not well adapted to respond to impacts from all directions, necessitating use of more than one sensor in automotive applications.
U.S. Pat. No. 4,827,091 discloses an impact sensor in which a permanent magnet is longitudinally movable within a cavity of a non-magnetic body. The magnet is biased toward one end of the cavity by magnetic attraction to a ring positioned externally of the cavity. Electrical switch contacts are positioned at the opposing end of the cavity for abutting electrical and mechanical engagement with the magnet. U.S. Pat. No. 4,484,041 discloses an impact sensor in which a permanent magnet is movably mounted within a cavity of a non-magnetic body, and normally held by magnet attraction against a magnetically permeable element that closes one end of the cavity. Application of acceleration forces to the body sufficient to overcome the force of magnetic attraction propels the magnet against a spring at the opposing end of the cavity, which returns the magnet to its normal position when the acceleration forces are removed. A reed switch is positioned adjacent to the cavity to detect motion of the magnet against the spring.
U.S. Pat. No. 4,639,563 discloses a sensor in which a pair of magnets are movably mounted within the cavity of a nonmagnetic body and oriented such that like poles on the magnets are opposed to each other and hold the magnets against opposing ends of the cavity. A reed switch is positioned adjacent to the cavity and generates an output signal when acceleration forces on either magnet are sufficient to overcome the force of magnetic repulsion and move the magnet toward the center of the cavity.
U.S. Pat. No. 5,177,370 discloses an acceleration sensor that comprises a body of non-magnetic construction having a linear internal cavity of uniform circular cross section. At least one permanent magnet is movably mounted within the cavity, and is resiliently urged toward one end of the cavity in such a way that acceleration forces on the sensor move the magnet toward the opposing end of the cavity. At least one Wiegand wire is positioned externally of the cavity between the cavity ends. The Wiegand wire is characterized by two stable magnetic flux-generating states dependent upon application of an external magnetic field of appropriate polarity for switching between such states. An electrical coil is positioned adjacent to the Wiegand wire, and is responsive to switching between the two flux-generating states for generating a sensor output signal as a result of acceleration forces on the magnet sufficient to overcome the force urging the magnet toward the one cavity end, and thus to bring the magnet into proximity with the Wiegand wire.
A general object of the present invention is to provide an acceleration sensor that has particular utility as an impact sensor for activating a vehicle restraint system, that is compact and rugged in construction, that is reliable in operation, and that can be produced at relatively low cost employing conventional manufacturing technology. Yet another object of the present invention is to provide a vehicle restraint system that employs such a sensor.